How to get into bones: proton pump and carbonic anhydrase inOsedaxboneworms

Abstract

Osedax are gutless siboglinid worms that thrive on vertebrate bones lying on the ocean floor, mainly those of whales. The posterior body of female Osedax penetrates into the bone forming extensions known as 'roots', which host heterotrophic symbiotic bacteria in bacteriocytes beneath the epidermis. The Osedax root epithelium presumably absorbs bone collagen and/or lipids, which are metabolized by the symbiotic bacteria that in turn serve for Osedax's nutrition. Here, we show that Osedax roots express extremely high amounts of vacuolar-H þ -ATPase (VHA), which is located in the apical membrane and in cytoplasmic vesicles of root and ovisac epithelial cells. The enzyme carbonic anhydrase (CA), which catalyses the hydration of CO 2 into H þ and HCO 3 2 , is also expressed in roots and throughoutOsedax body. These results suggest Osedax roots have massive acid-secreting capacity via VHA, fuelled by H þ derived from the CA-catalysed hydration of CO 2 produced by aerobic metabolism. We propose the secreted acid dissolves the bone carbonate matrix to then allow the absorption of bonederived nutrients across the skin. In an exciting example of convergent evolution, this model for acid secretion is remarkably similar to mammalian osteoclast cells. However, while osteoclasts dissolve bone for repairing and remodelling, the Osedax root epithelium secretes acid to dissolve foreign bone to access nutrients.

Novel and potential physiological roles of vacuolar-type H+-ATPase in marine organisms

“…Nonetheless, immunohistochemistry and western blots on samples enriched in gill cell membranes from control and base-infused hagfish suggest that VHA is present both in cytoplasmic vesicles and in the basolateral membrane, and that -similar to results from sharks -VHA inserts into the basolateral membrane and corrects blood alkalosis (Tresguerres et al, 2007a). Cells with basolateral VHA are also found in gills from squid (Hu et al, 2014), crab and bone-eating Osedax worms (Tresguerres et al, 2013) (see below). However, it is not known whether VHA energizes HCO 3 − secretion in any of these cells.…”

“…The posterior end of Osedax branches out and penetrates deep into the bone, resembling the roots of a tree, and these roots host symbiotic heterotrophic bacteria. Osedax worms derive their nutrition from collagen and lipids trapped in the bones (Goffredi et al, 2005;Rouse et al, 2004), which they release by secreting acid using VHA that is present at high levels in the apical membrane of root epithelial cells (Tresguerres et al, 2013). The proposed Osedax feeding mechanism is a multistep process that starts with VHA-mediated secretion of H + onto the bone, which dissolves the calcium phosphate matrix and releases collagen and lipids (Fig.…”

“…In order to produce H + , CO 2 is hydrated to H + and HCO 3 − by CA. The CO 2 is generated by aerobic metabolismOsedax has a robust circulatory system that ensures sufficient O 2 delivery to the root epithelium (Huusgaard et al, 2012), and 20 µm epithelial cells that are rich in mitochondria and CA (Tresguerres et al, 2013). This implies that as H + is secreted onto the bone, an equimolar amount of HCO 3 − is secreted into the worm's tissues.…”

Ammonia excretion in mytilid mussels is facilitated by ciliary beating

“…For detection of the VHA B-subunit, custom-made purified polyclonal rabbit antibodies against the peptide AREEVPGRRGFPGY were used ( protein size 55 kDA). These antibodies specifically recognize VHA B in a variety of diverse organisms including coral, bone-eating worms and sharks (Tresguerres et al, 2013;Roa et al, 2014;Barott et al, 2015). Additionally, commercially available polyclonal rabbit antibodies against mammalian α-subunit of sodium/potassium ATPase (NKA; α300, Santa Cruz Biotechnology, Dallas, TX, USA) and human carbonic anhydrase II (CA II; Rockland, Gilbertsville, PA, USA) were used recognizing proteins of the expected sizes of approximately 116 and 36 kDa, respectively.…”

Genomic versatility and functional variation between two dominant heterotrophic symbionts of deep-sea Osedax worms

“…Collagen from bone appears to be their nutritional foundation, based on d 13 C signatures that are consistent between Osedax tissues and the collagen-specific fraction in whale bones (Goffredi et al, 2005). The worms are hypothesized to use acidifying vacuolar H þ -ATPases in their root-like tissue to bore into and breakdown the hard, calcified matrix of bones (Tresguerres et al, 2013). Based on unsuccessful attempts at PCR amplification of genes required for autotrophic CO 2 fixation (that is, ribulose-1, 5-bisphosphate carboxylase/oxygenase and ATP citrate lyase), the symbionts are hypothesized to be uniquely heterotrophic, rather than chemosynthetic, as in all other examined siboglinids (Goffredi et al, 2005).…”